Assembly for use in orthopaedic surgery

ABSTRACT

An assembly for use in orthopaedic surgery comprises a component which is to be positioned within a body cavity to engage a bone the component comprises a hollow shell which is open on one side to allow access to its interior and has a bar extending across it. The assembly includes a manipulator having a clasp for engaging the bar so as to fasten the component to the manipulator, in which the clasp allows rotation of the bar so that the angular orientation of the component relative to the manipulator can be changed.

This invention relates to an assembly for use in orthopaedic surgery,which comprises a hollow shell component having a bar extending acrossit, and a manipulator having a clasp for engaging the bar so as tofasten the component to the manipulator.

Hollow shell components have uses in orthopaedic surgery such as shapingbone tissue, for example to receive a component of an orthopaedic jointprosthesis, and as orthopaedic joint prosthesis components. Aninstrument having a hollow shell configuration has to be manipulatedwhen used to shape a bone. For example, when the instrument is a cuttingtool with cutting teeth on its external surface, it can be rotated aboutan axis of symmetry (for example when the external surface defines partof a sphere) to cause the bone tissue to be cut. A component of a jointprosthesis has to be manipulated to ensure that it is aligned properlywith the prepared surface of the bone.

It is known to provide a hollow shell component with a bar which extendsacross it which can be engaged by a manipulator with an appropriateclasp. In the case of an instrument, the bar does not need to bedetached from the shell, and can be bonded to the shell or formedintegrally with it, for example by casting. In the case of a componentof a joint prosthesis, the bar can be attached to the shell component bymeans of appropriate formations, for example which engage a lip on thecomponent.

It is desirable to minimise the size of the incision that is necessaryduring surgery, for example to minimise blood loss and damage to softtissue, as well as for aesthetic reasons. When performing surgery on apatient's hip joint, especially when implanting an acetabular cupprosthesis, it is generally the case that the incision has to be capableof accommodating the cup prosthesis itself when directed towards theacetabulum, aligned appropriately relative to the relevant axis.

The present invention provides an assembly for use in orthopaedicsurgery which comprises a shell component and a manipulator, in which aclasp on the manipulator engages a bar on the shaft, so as to allowrotation of the bar within the clasp.

Accordingly, in one aspect, the invention provides an assembly for usein orthopaedic surgery, which comprises a component which is to bepositioned within a body cavity to engage a bone, the componentcomprising a hollow shell which is open on one side to allow access toits interior and which has a bar extending across it, and a manipulatorhaving a clasp for engaging the bar so as to fasten the component to themanipulator, in which the clasp allows rotation of the bar so that theangular orientation of the component relative to the manipulator canchange.

The assembly of the present invention has the advantage that, for agiven size of shell component and a given manipulator, the component canbe delivered to the relevant bone through a smaller incision than mightbe necessary using known assemblies, by changing the angular orientationof the component relative to the manipulator.

Preferably, the bar which has a generally rounded cross-section.Preferably, the clasp has a generally rounded recess in which the barcan be received. These features can facilitate rotation of the barwithin the clasp.

Preferably, the clasp comprises a recess which is shaped to receive thebar, and a locking part which can be moved between two positions inwhich (a) the bar is prevented from moving out of the recess, and (b)the bar can be moved out of the recess, respectively. For example, therecess can be approximately C-shaped, so that the bar is received in therecess by being slid transversely. A retractable pin can then close therecess, which allows the bar to be moved out of the recess whenretracted. The pin can be profiled so that it can be displaced by thebar when force is applied to the bar to force it into the recess.

Alternatively, cooperating formations on the clasp and the head canengage when the bar is received in the recess to prevent the bar frombeing withdrawn from the recess. For example, a protruding formationsuch as a ridge or pin can be received in an appropriate recessformation (which might be a groove when it is intended to receive aprotruding formation in the form of a ridge).

Preferably, the clasp includes at least two recesses. For example, whenthe bar extends across the shell, the clasp can include two recesses toengage the bar on opposite sides of the centre. Preferably, each recesshas a respective locking part, for example comprising a protrudingformation and a recess formation.

Preferably the movable locking parts of the clasp which cooperate withrespective recesses for the bar can be moved together between the lockedand released positions. For example each of the locking parts can beprovided on a sliding collar.

Preferably, the clasp has a lock which can engage the bar to restrictmovement of the bar relative to the clasp. Preferably, the lock can bemoved between a first position in which the bar can move relative to themanipulator and a second position in which movement of the bar isrestricted. The movement which is restricted by the lock can involve (a)rotation of the component about the axis defined by the bar when it isreceived within the clamp, or (b) movement of the component relative tothe manipulator during assembly of the component on the manipulator.

For example, the bar can have at least one aperture in it, and the claspcan include a retractable pin which can be received in the aperture inthe bar to lock the bar against rotation relative to the clasp. The barcan have a flat on one side (or more than one flat, for example twoflats on opposite sides) and the lock can comprise a C-shaped collarwhich a flat side, which can only fit on to the bar when the flat on thebar and the flat side on the collar are aligned. Preferably, the lock isbiassed towards the position in which it restricts rotation of the bar.The lock (for example the pin or the C-shaped collar) can be mounted ona collar which can slide relative to the clasp. The lock can be providedon the same collar as locking parts by which the bar is retained withina recess of the clasp.

A preferred lock comprises at least one recess which is provided in oneof the component and the manipulator, and at least one ridge which isprovided in the other of the component and the manipulator. When therelative rotational positions of the component and the manipulator aresuch that the ridges are aligned with the recesses the ridges can bereceived in the recesses. Further rotation of the component relative tothe manipulator is not possible while the ridges are received in thegrooves.

Preferably, the assembly includes an actuator for causing the angularorientation of the component relative to the manipulator to change. Theactuator can comprise an actuator part on the manipulator which can bemoved relative to the clasp. The actuator can include a hook which isprovided on one of the component and the actuator part, and a recesswhich is provided in the other of the component and the actuator part.The angular orientation of the component can be changed by moving theactuator part relative to the clasp, which causes the component to moveby virtue of the hook being received in the recess.

The actuator part can comprise a sleeve which can be slid relative to aclasp head, by which the bar is engaged by the manipulator. The sleevecan provide a recess which is defined by a hook, which can receive ahook on the component beneath it. When the hook on the component isreceived in the recess on the sleeve, sliding the sleeve relative to theclasp head can cause the angular orientation of the component relativeto the manipulator to change, by rotation of the component about theaxis defined by the bar, engaged within the clasp on the manipulator.

The sleeve can carry formations which engage corresponding formations onthe component, to lock the component in a desired orientation. Forexample, ridges or recesses on the manipulator, which fit intocorresponding recesses or ridges on the component, can be provided onthe sleeve.

Preferably, the shell has a cut out portion towards the open side in aregion of its wall that is located approximately opposite to themidpoint of the bar. This can be of particular advantage when the barcan rotate relative to the clasp because it can reduce interference ofthe manipulator with rotation of the component and therefore allowrotation of the component through a larger angle.

The external surface of the shell can be symmetrical about an axis ofrotation. For example, the external surface can define a part of asphere. This might be the case when the assembly is for use inconnection with a joint prosthesis in which one component articulatesagainst the other component in the manner of a ball which is received ina cup. While the configuration of the external surface can preferably besymmetrical about an axis of rotation, the component need not besymmetrical in this way. For example, the shell can have one or morecut-out portions. The wall thickness of the shell can vary from oneregion to another. The shell can have features on its outer surfaceaccording to its intended purpose: for example, the component might be acutting tool, in which case it can have cutting teeth on its outersurface, and the arrangement of the teeth on the surface need notnecessarily be symmetrical about the axis of symmetry.

When the component is a cutting tool, the use of a rotationallysymmetrical component has the advantage that the tool can be rotatedabout the axis to cut the patient's bone tissue. When the externalsurface of the component defines a part of a sphere, the tool can berotated about its axis to cut the patient's bone tissue, while at thesame time the orientation of the axis relative to the bone is changed.This can be important to achieve satisfactory cutting of the patient'sbone in preparation for implantation of a joint prosthesis component.

Preferably, the bar to which the clasp fastens intersects the axis ofsymmetry.

Preferably, the portion of the bar which is engaged by the clasp islocated within the shell so that, when the component is fastened to themanipulator, the clasp is located at least partially within the shell.

Preferably, the ratio of the depth of the centre of the bar within theshell measured from the open side of the shell, to the length of theaxis measured from the open side of the shell to the external surface ofthe shell opposite the open side, is at least about 0.2. When the barintersects the axis of symmetry of the component, the depth of the baris measured from the open side of the shell to the point where thecentre of the bar intersects the axis of symmetry. When the bar does notintersect the axis of symmetry, the depth of the bar is measured alongthe axis, to the point at which the centre of the bar is closest to theaxis. Preferably, the value of the said ratio is at least about 0.4,more preferably at least about 0.5.

The bar can be straight (when viewed along a line perpendicular to theaxis of the shell and to the bar), in which case, it will be fastened tothe internal wall of the shell at its ends, at the same depth as pointat which the clasp engages the bar. The bar can be cranked, so that thedepth of the ends of the bar within the shell need not be the same asthe depth of the point at which the clasp engages the bar. For example,the ends of the bar can be located closer to the open side of the shellthan the point at which the clasp engages the bar, for example with theends of the bar fastened to the shell at the open side. Preferably, thebar is approximately straight in the region thereof in which it isengaged by the clasp. The ends of the bar can be located further fromthe open side of the shell than the point at which the clasp engages thebar; for example the ends of the bar can be fastened to the internalsurface of the shell close to the pole of the shell. The bar can also bemounted on a fixture which is fastened to the internal wall of the shellat or close to the pole: for example, the fixture can comprise a lengthof a tube, and the bar extends across the tube.

The bar is preferably straight when viewed along the axis of the shell(which is axis of symmetry when the shell is rotationally symmetrical).A bar can be considered as consisting of a plurality of limbs extendingfrom the polar axis of the component: for example, a single bar whichextends across the component from one side to the opposite side consistsof two limbs. The bar can include more than two limbs or there can bemore than one bar. For example, there can be two bars which are fastenedtogether at about the shell axis, so that there are four limbs extendingradially from the axis. The bars can then be perpendicular to oneanother at the point at which they are fastened together. Otherarrangements are envisaged, for example in which the bar is provided bythree limbs which are joined together so that the angle between any twoof the limbs is about 120°. The limbs can be joined together at or closeto the axis of the component. They might however not extend to the axisof the component and be joined together by means of a ring whichencircles the axis.

Preferably, the clasp comprises a recess which is shaped to receive thebar, and a locking part which can be moved between two positions inwhich (a) the bar is prevented from moving out of the recess, and (b)the bar can be moved out of the recess, respectively. Preferably, theclamp defines at least one recess which can present a transverse openingwhich allows the bar to be positioned in the recess by sliding ittransversely into the recess. The clamp can provide a plurality ofrecesses, corresponding to each limb of the bar or bars: for example,when there is one bar which extends across the component.

For example, the recess can be approximately C-shaped, so that the baris received in the recess by being slid transversely relative to theaxis of the component. The transverse sliding of the bar can involverelative translocation of the whole component relative to the clasp, orrelative rotation (especially without translocation) between thecomponent and the clasp in a plane which is transverse to the axis ofthe component, or both.

A retractable locking formation, such as a pin or a ridge, can close arecess, in which retraction of the pin allows the bar to be moved out ofthe recess. A locking formation can be profiled so that it can bedisplaced by the component when force is applied to the component toconnect it to the manipulator.

Preferably, the clasp includes at least two recesses. For example, whenthe bar extends across the shell, the clasp can include two recesses toengage the bar on opposite sides of the centre. When the bar has threelimbs, the clasp can present three recesses. The recesses can bearranged so that the clasp is rotationally symmetrical, facilitatingengagement of the component with the clasp by relative rotation. Eachrecess can have a respective locking part.

Preferably the movable locking parts of the clasp which cooperate withthe component to lock it, especially with respective recesses for thebar, can be moved together between the locked and released positions.For example each of the locking parts can be provided on a slidingcollar.

Preferably, the manipulator includes a tube portion and a shaft whichcan rotate within the tube portion, the clasp being fastened to theshaft so that the component can be rotated relative to the tube portion.The assembly can include a handle which is provided by or fastened tothe tube portion. The shaft can be driven by a rotary drive unit,especially when the component is a cutting tool.

Embodiments of the present invention will now be described by way ofexample with reference to the accompanying drawings in which:

FIG. 1 is an isometric view of a shell component of an assemblyaccording to the invention.

FIG. 2 is an exploded isometric view of an assembly according to theinvention.

FIG. 3 is an exploded side view of the assembly shown in FIG. 2.

FIGS. 4 to 6 are side views of an assembly according to the inventionwith the shell component in different angular orientations relative tothe manipulator.

FIG. 7 is an isometric view from below of a preferred embodiment ofreamer component.

FIG. 8 a is an isometric view of the clasp assembly of a manipulatorwhich can be used with the reamer component shown in FIG. 7.

FIG. 8 b is another isometric view of the clasp assembly shown in FIG. 8a, viewed along the arrow “VIII”.

FIG. 9 is an exploded view of the manipulator shown in FIGS. 7 and 8.

FIGS. 10 a and 10 b are sectional elevations from one side of the reamercomponent and manipulator shown in FIGS. 7 and 8 respectively, in twoangular orientations.

Referring to the drawings, FIG. 1 shows a reamer component of aninstrument set which can be used in the implantation of the acetabularcup component of a hip joint prosthesis. The component comprises ahollow shell 2 formed from a suitable metallic material (for example astainless steel) which has raised cutting teeth 4 arranged on its outersurface. The external surface of the shell defines a part of a sphereand the open side of the shell is at or slightly above about theequatorial plane of that sphere, so that the shell is symmetrical aboutthe polar axis of the shell. Each of the cutting teeth is directed so asto cut the surface of a bone when rotated about the axis of symmetry.The shell is provided by a thin sheet of the metallic material, and canbe manufactured by forming a sheet, or by other techniques such ascasting and appropriate subsequent finishing.

The reamer component shown in FIG. 1 has a bar 6 within it. The bar isfastened at its ends to the internal surface of the shell, for exampleby welding. The bar is straight, when viewed along the axis of thesphere, and also when viewed from one side.

The bar 6 in the reamer head has a plurality of lock holes 8 within itat its centre point. The shell has a cut out portion 10 in its outerwall.

FIGS. 2 and 3 show an assembly according to the invention whichcomprises a reamer head 50 and a manipulator 52. The manipulatorcomprises a proximal portion 54 with which the assembly can be held. anda distal portion 56. The angle between the proximal and distal portionsis about 135°. Each of the proximal and distal portions is hollow and adrive shaft 58 extends through them, via a universal joint at the pointwhere the portions are joined.

The reamer head has a single bar 60 within it, extending between theopposite internal surfaces of the head.

The manipulator includes a clasp head 62 which has four recesses 64 init. Each of the recesses has an opening 66 on the face of the clasp headwhich is directed into the shell of the reamer head. The clasp head hasa hole 68 extending through it associated with each of the recesses 64.A collar 70 which can slide relative to the clasp head along the distalportion of the manipulator has four closure pins 72 on it, directedalong the axis of the distal portion, so that they can slide into andout of the holes 68 in the clasp head. The collar is resiliently biassedtowards the clasp head by means of a spring acting against a biassingface 74.

One of the recesses 64 in the clasp head also has a lock pin hole 76associated with it, extending through the clasp head into the opening.The collar 70 has a lock pin 78 on it, again directed along the axis ofthe distal portion, so that it can slide into and out of the lock pinhole in the clasp head. The lock pin 78 is shorter than the closure pins72.

The bar 60 in the reamer head has a lock hole 80 within it towards oneend thereof. This can be contrasted with the embodiment shown in FIG. 2in which a lock hole is located centrally on the bar. The shell has acut out portion 82 in its outer wall.

In order to fasten the reamer head to the clasp head, the collar 70 isretracted along the distal portion 56 to withdraw the closure pins 72and the lock pin 78 from their respective holes 68, 76 in the clasphead. The bar 60 in the reamer head is then inserted into the openings66 in the clasp head which communicate with the recesses 64, and twistedrelative to the clasp head so that the bar is received firmly within therecesses. Once the bar has been twisted clear of the openings 66, thecollar 70 can be released so that it moves outwardly along the distalportion of the retractor, so that the closure pins 72 extend from theirrespective holes 68 in the clasp head, preventing the bar frominadvertently twisting out of the recesses.

When the lock pin hole 76 is aligned with the axis of the distal portion56 of the manipulator, the collar can slide fully towards the reamerhead so that the lock pin 78 extends through the lock pin hole 76 in theclasp head and into the lock hole 80 in the bar 60. This preventsrotation of the bar (and the reamer head) relative to the clasp and themanipulator. When the lock pin hole 76 is not aligned with the axis ofthe distal portion of the manipulator, the lock pin 78 is not able toextend from the lock pin hole in the clasp head, and this preventsmovement of the collar fully towards the reamer head. However, becausethe lock pin 78 is shorter than the closure pins 72, the closure pinsstill serve to prevent the bar 60 from being moved out of the recessesin the clasp head.

The cut out portion 82 in the other wall of the reamer head allows thehead to be rotated so that the plane containing the open side of thehead is substantially parallel to the axis of the distal portion of themanipulator.

FIGS. 4 to 6 show an assembly of the invention in which the shellcomponent can rotate relative to a clasp on the manipulator, and inwhich there are a number of defined orientations of the component, forexample by means of a plurality of lock holes as in the shell componentshown in FIG. 1.

FIG. 4 shows the shell component with the plane defined by the open sideof the shell 30 component parallel to the axis 32 of the manipulator 34.FIG. 5 shows the shell component with angle between the plane defined bythe open side of the shell component and the axis of the manipulatorequal to about 25°. FIG. 6 shows the shell component with angle betweenthe plane defined by the open side of the shell component and the axisof the manipulator equal to about 90°.

It will be common for the reamer head shell component to be deployed foruse in the arrangement shown in FIG. 6. However, it is readily apparentfrom comparison of FIGS. 4 to 6 that the size of the incision that isnecessary to locate the reamer in the relevant body cavity is greaterwhen the assembly is in that configuration than when in either of theconfigurations shown in FIGS. 4 and 5. The present invention makes itpossible for the reamer head to be inserted into the body cavity in asmall size configuration as shown in FIGS. 4 and 5, and then to bedeployed for use in the FIG. 6 configuration once within the cavity.

A cut-out portion in the wall of the shell allows the shell to move fromthe FIG. 5 configuration to the FIG. 4 configuration, where the handleof the manipulator fits into the cut out portion in the wall of theshell.

FIG. 7 shows a reamer component of an instrument set which comprises ahollow shell 102 with raised cutting teeth 104 on its outer surface. Thecomponent has a bar 106 within it, fastened at its ends to the internalsurface of the shell, for example by welding. The bar is straight whenviewed along the axis of the sphere and also when viewed from one side.

The bar has two collars 108 mounted on it, towards respective oppositeends of the bar. Each of the bars has a groove 110 formed into it. Thecollar has a circular shape when viewed along the bar and the grooveextends radially relative to the circular cross-section of the collar.The groove extends from the surface of the rod in a direction which isparallel to the polar axis of the shell. The rod and its collars can beformed by casting, or from separate parts which are connected together,for example by welding or by means of adhesive, or mechanically etc.

The bar has a hook 112 mounted on it, in the illustrated embodiment atabout the midpoint along its length. The hook is turned upwardly towardsits free end, generally towards the pole of the shell.

FIGS. 8 a and 8 b show the clasp assembly of a manipulator which issimilar to that shown in FIGS. 2 and 3. The clasp assembly includes aclasp head 118 which has two recesses 120 in it, each of them having anopening 122. The recesses are arranged so that the bar can be retainedin the clasp, under two opposites ones of the recesses.

The clasp includes a retaining sleeve 124 which can be moved relativethe clasp head 118 along the axis of the clasp. The retaining sleeve hastwo upwardly facing ridges 126 (of which one only is visible in theFIGS. 8 a and 8 b). The ridges are aligned with two of the recesses.

The retaining sleeve also presents a hook 128 which is located on theperiphery of the sleeve, on a line which extends approximatelyperpendicular to a line which joins the two ridges 126.

FIG. 9 shows the internal parts of a manipulator in an exploded view,including the clasp head 118 and the retaining sleeve 124. The drawingdoes not include an external sleeve which can be fitted over theinternal parts, for example to allow the instrument to be gripped, andto isolate parts of the instrument which move when the instrument is inuse.

The manipulator includes a drive shaft 130 by which rotational drive canbe imparted to a reamer or other component fastened to the claspassembly. The drive shaft can be arranged at an angle to the axis of thecomponent. Drive to the component is transmitted through a universaljoint 132 of a conventional kind, which is connected directly to thebase 134 of the clamp assembly.

The clamp assembly includes a spring 136 which acts on between retainingsleeve 124 at the upper end of the spring and a collar 138 at its lowerend. The collar fits onto the base of the clamp assembly where it isfastened, for example by means of a screw thread or bayonet fixingformations. The spring is therefore able to urge the retaining sleeveupwardly relative to the clamp head, and to allow the retaining sleeveto be displaced reversibly in a downward direction.

The retaining sleeve 124 can be moved downwardly relative to the clasphead. This can be done by grasping the retaining sleeve itself.Alternatively, it can be made to move by remote control, for example bymeans of an actuator which engages the retaining sleeve. The retainingsleeve has a peripheral groove 140 towards its upper end by which it canengage an actuator for axial movement thereof.

A reamer component (such as that shown in FIG. 7) is connected to themanipulator shown in FIGS. 8 and 9 by locating the bar 106 adjacent tothe openings 122 into the recess 120. Rotation of the component headrelative to the clasp head causes the bar to be received within therecesses.

The collars 108 on the bar 106 act against the upwardly facing surfaceof the retaining sleeve 124 on which the ridges 126 are provided. As thereamer component is rotated relative to the clasp head, the collars moveacross the said upwardly facing surface towards the ridges. Action ofthe collar against the ridges causes the retaining sleeve to bedisplaced downwardly against the action of the spring 136 as it iscompressed. When the relative rotational positions of the component andthe clasp head are such that the ridges are aligned with the grooves 110in the collars 108, the retaining sleeve moves upwardly so that theridges are received in the grooves. Further rotation of the reamercomponent relative to the clasp head is not possible while the ridgesare received in the grooves.

As the reamer component is rotated relative to the clasp head, the hook112 on the bar 106 moves under the hook 128 on the retaining sleeve 124.

FIG. 10 a shows a reamer component mounted on the head of themanipulator, with the bar 106 received in the recesses on the clasp headand the grooves on the retaining sleeve received in the grooves in thecollars on the bar. The hook 112 on the bar 106 is received under thehook 128 on the retaining sleeve 124. The reamer component is then inits position for use, in which rotational drive can be imparted to itthrough the drive shaft 130.

The reamer component can be tilted by axial withdrawal of the retainingsleeve 124, as shown in FIG. 10 b. Location of the hook 112 on the bar106 under the hook 128 on the retaining sleeve causes the reamercomponent to be tilted, about the axis defined by the bar 106 within therecesses on the clasp head.

The reamer component is released from the clasp head by partial axialwithdrawal of the retaining sleeve, sufficient to withdraw the ridges onthe retaining sleeve from within the grooves on the collars, but notsufficient to cause the hook 112 on the bar 106 to engage the hook 128on the retaining sleeve and thereby to cause the reamer component totilt. This then allows the reamer component to be rotated relative tothe clasp head so that the bar can be released from within the recesses.

The assembly of the present invention can be used to manipulate othercomponents. For example, it can be used to manipulate instruments otherthan reamers. It can be used to manipulate implant components, forexample the acetabular cup component of a hip joint prosthesis.Generally, the prosthesis will require a suitable mounting for thefastening formation.

1. An assembly for use in orthopaedic surgery, which comprises acomponent which is to be positioned within a body cavity to engage abone, the component comprising a hollow shell which is open on one sideto allow access to its interior and which has a bar extending across it,and a manipulator having a clasp for engaging the bar so as to fastenthe component to the manipulator, in which the clasp allows rotation ofthe bar so that the angular orientation of the component relative to themanipulator can change.
 2. An assembly as claimed in claim 1, in whichthe clasp has a lock which can engage the bar to restrict rotation ofthe bar relative to the clasp.
 3. An assembly as claimed in claim 2, inwhich one of the bar and the clasp presents at least one ridge, and theother of the bar and the clasp presents a corresponding groove, arrangedso that the ridge can be received in the groove to restrict rotation ofthe bar relative to the clasp.
 4. An assembly as claimed in claim 2, inwhich the bar has at least one aperture in it, and in which the claspincludes a retractable pin which can be received in the aperture in thebar to lock the bar against rotation relative to the clasp.
 5. Anassembly as claimed in claim 1, which includes an actuator for causingthe angular orientation of the component relative to the manipulator tochange.
 6. An assembly as claimed in claim 5, in which the actuatorcomprises an actuator part on the manipulator which can be movedrelative to the clasp, and in which the actuator includes a hook whichis provided on one of the component and the actuator part, and a recesswhich is provided in the other of the component and the actuator part,and in which the angular orientation of the component can be changed bymoving the actuator part relative to the clasp, which causes thecomponent to move by virtue of the hook being received in the recess. 7.An assembly as claimed in claim 1, in which the shell has a cut outportion towards the open side in a region of its wall that is locatedapproximately opposite to the midpoint of the bar.
 8. An assembly asclaimed in claim 1, in which the manipulator includes a tube portion anda shaft which can rotate within the tube portion, the clasp beingfastened to the shaft so that the component can be rotated relative tothe tube portion.
 9. An assembly as claimed in claim 1, in which theexternal surface of shell is symmetrical about an axis of rotation. 10.An assembly as claimed in claim 9, in which the bar intersects the axisof rotational symmetry.
 11. An assembly as claimed in claim 9, in whichthe external surface of the shell defines a part of a sphere.
 12. Anassembly as claimed in claim 1, in which the portion of the bar which isengaged by the clasp is located within the shell so that, when thecomponent is fastened to the manipulator, the clasp is located at leastpartially within the shell.
 13. An assembly as claimed in claim 12, theexternal surface of shell is symmetrical about an axis of rotation, andin which the bar intersects the axis of rotational symmetry, the ratioof the distance from where the axis intersects the open side of theshell to the centre of the bar where it intersects the axis, to thelength of the axis measured from the open side of the shell to theexternal surface of the shell opposite the open side being at leastabout 0.2.
 14. An assembly as claimed in claim 1, in which the bar isstraight.
 15. An assembly as claimed in claim 1, in which the bar iscranked.
 16. An assembly as claimed in claim 1, in which the componentis a cutting tool, in which the external surface of the shell hascutting teeth.
 17. An assembly as claimed in claim 1, in which thecomponent is a component of an orthopaedic joint prosthesis.